1. Field of the Invention
The invention relates to a method and a device for the spatially resolved detection and reconstruction of objects by means of microwaves. In this context, at least one object to be detected is subjected to microwaves that are generated by a plurality of microwave antennas and at least fractions thereof are reflected by the object. These reflected microwave fractions are detected and converted into microwave signals that can be analyzed, based on which analysis for the spatially resolved object detection is carried out. An analysis technique of this kind is very suitable for security examinations, for example of people at border crossings. For example, it is possible to use this microwave technique to screen persons for hidden objects such as weapons, explosives and other such materials.
2. Description of the Prior Art
In today's globally interconnected world, security considerations are becoming more and more important. The most widely employed security systems for examining items of luggage, cargo or people, such as are used for example in airports and railway stations, rely on X-rays to detect dangerous objects and substances. Examinations of this kind are typically based on the simple principle of radiation, in which materials of different density are differentiated according to their individual contrast characteristic in the X-ray projection image.
An alternative method uses electromagnetic waves in the microwave range, that is to say electromagnetic waves in the frequency range from 300 MHz to 200 GHz. In this context, microwave fields strike an object, substance or person to be examined, that is to say any object, and are reflected back differently depending on the nature of the object. Unfortunately, the resolution capability of a conventional microwave method is limited by its relatively long wavelength, but this drawback may be overcome by the use of tomographic measurement and reconstruction principles, which also enable objects to be reconstructed with a spatial resolution that under certain circumstances may even extend beyond the wavelength boundary.
The advantages of such a microwave method, abbreviated to MW-method, compared to the widely used X-ray method, are obvious. On the one hand, ionizing radiation is used, on the other hand, with the imaging-forming object reconstruction provided by tomographic analysis techniques it is possible to obtain spatial information about the objects being examined.
Previously known and commercially available MW systems normally use arrays of sensors to scan the objects, which are attached to a moving gantry. Scanning creates a synthetic aperture that enables spatial focusing in the volume.
However, apparatuses of this kind are extremely complex in mechanical terms, and expensive to build. The examination also takes a long time, so the practical utility of such systems, particularly in examination areas with high traffic rates, such as airport security check lines, are in need of improvement. In addition, the object must remain motionless during the screening, a condition that is practically impossible, or at least very difficult to achieve, particularly when people are being examined.
A method and device for measuring multiphase flows in tubes are disclosed in EP 0 703447 A2. In this context, the distributions of permittivity and conductivity relative to the tubular cross-section are detected via a plurality of microwave antennas arranged around the tube.
A method for dielectric scanning of a person is known from U.S. Pat. No. 6,927,691 B2. In this context, the person is radiated with electromagnetic radiation at a frequency of 5.5 GHz. Anomalies such as caused, for example, by weapons or contraband may be detected and displayed by comparison of the captured values with an expected dielectric response.
Surveillance systems based on a holographic reconstruction of an examined object are known from US Publications 2004/0090359 A1 and US 2005/0232459.